Decision-making regarding breast cancer adjuvant chemotherapy in patients with favorable prognoses poses challenges for clinicians and patients alike given the relatively low marginal benefit of chemotherapy. Genomic profile assays in early stage, estrogen receptor-positive breast cancer refine estimates of both the risk of distant recurrence and the marginal benefit of chemotherapy over endocrine therapy alone and thereby enhance individualized care for patients with cancer. The benefits of the assay, however, will be realized only if the assay is used in the right patients and if receipt of chemotherapy is guided by assay results. The extent to which genomic profiling assays are used in practice and the impact of the assay results on receipt of chemotherapy in population-based patient samples are unknown. The aims of this study are to investigate patient and clinician factors associated with the use of genomic profiles, specifically the 21-gene assay, and the impact of the assay on receipt of chemotherapy in at population-based sample. In addition, the study findings will inform innovative dissemination strategies of our research findings among oncologists. To achieve these aims, we will conduct a prospective population-based cohort study of 3,200 patients with early stage invasive, estrogen receptor-positive breast cancer and their treating oncologists (approximately 270) in the state-wide New Jersey and Georgia Surveillance, Epidemiology, and End Results (SEER) registries. Patients who are candidates for the genomic assay and their oncologists will be surveyed several months after diagnosis. Patient-reported variables, oncologist knowledge and attitudes elicited through vignettes, practice characteristics obtained from the oncologist survey, and SEER data will be merged to create a unique dataset. Multilevel analyses will incorporate consideration of patient, clinician, and practice influences on use of the assay and receipt of chemotherapy. The project aims will address critical barriers to our understanding of use and diffusion of tests-not only in breast cancer-^that promote individualized care and will ultimately help guide policies and interventions aimed at improving care.